Read through perlunicode. All your I/O operations need to be performed in UTF-8. That means not only open (STOPWORDS, '<:encoding(UTF-8)', $ARGV[1]) as ForgotPasswordAgain suggests and open (INFILE, '<:encoding(UTF-8)', $ARGV[0]) as derby suggests, but also binmode STDOUT, ":encoding(utf8)" before you try to print. The fact that it works with "standard" text says it is almost guaranteed to be a Unicode problem.